1. Field of the Invention
The present invention relates to an input device, and more particularly to an input device operable in a power saving mode in which a microcomputer is shut down in a standby state.
2. Description of the Related Art
An electronic device such as an AV device has an operation state, called a “standby state”, where power is supplied only to the microcomputer and to the peripheral circuits thereof so that the power of the device can be turned back ON in response to a user's operation of a key or a remote control even when the power of the device is OFF. Recently, there is a demand for further reducing the standby power consumption, i.e., the amount of power to be consumed in a standby state. In view of such a demand, the microcomputer has a power saving mode in which the microcomputer itself is shut down when the microcomputer has no process to execute. Then, in response to a user's operation of a key, an interrupt signal is input to an interrupt terminal of the microcomputer to exit the power saving mode (Japanese Patent Laid-Open No. 9-198169 and Japanese Patent Laid-Open No. 7-13671). However, Japanese Patent Laid-Open No. 9-198169 requires the provision of an OR circuit and a general-purpose input terminal for each key switch, thereby complicating the circuit configuration. In Japanese Patent Laid-Open No. 7-13671, various keys of a key matrix are scanned repeatedly so as to detect a valid key input from the key matrix. However, the scanning signal is a cause of noise, and such a configuration is inappropriate particularly in AV applications.
FIG. 5 shows a conventional circuit 501 with which a microcomputer 2 can determine which one of a plurality of key switches connected in parallel is turned ON with a single key input terminal a. With the circuit 501, when a key switch S2 is turned ON, for example, a voltage of V1×R2/(R1+R2) is input to the key input terminal a of the microcomputer 2. The input voltage is A/D-converted and compared with predetermined voltage values associated with key switches S1 to S5 so that the microcomputer 2 can determine that the key switch S2 has been pressed.
In order to provide the microcomputer 2 of the circuit 501 with a power saving mode, it is necessary to provide a comparator 505 for the transition from the power saving mode to the normal mode. The voltage at the node A is input to one end of the comparator 505, and a reference voltage is input to the other end of the comparator 505. When a key switch is turned ON, the comparator 505 applies an interrupt signal (a low-level voltage) to an interrupt terminal b. With the circuit 501, however, it is necessary to provide a comparator so as to be able to exit the power saving mode, thus increasing the cost. In order to solve the problem, Japanese Patent Laid-Open No. 2001-186657 discloses a method in which the microcomputer is operated intermittently using a built-in timer function in a standby state, and the A/D converter is operated periodically, so as to determine which key switch is turned ON. This method eliminates the need for a comparator. However, since the microcomputer is operated intermittently in a standby state, the standby power consumption cannot be reduced sufficiently.